In recent years, the public has become increasingly aware of the deteriorating quality and quantity of our nation's and the world's fresh water supply. Pollutants, biological and toxic waste and other contaminants are being introduced into water supplies at an ever increasing rate, making such water supplies unfit for drinking and other necessary uses. For example, medical patients with low immunity are now being requested not to drink tap water, and disease and illnesses linked to poor quality drinking water have increased dramatically in recent years. This problem is especially significant outside the United States where water quality has deteriorated to an all time low, with the major source of such contamination primarily being bacterial in nature.
In many areas of the world potable water is not only contaminated but it is also scarce. In these areas people must rely upon expensive purification systems to remove dissolved solids from sea water or well water.
Reverse osmosis filtration systems are some of the most common solutions for improving water quality. Osmosis is the flow or diffusion that takes place through a semipermeable membrane (as in a living cell) typically separating either a solvent (as water) and a solution or a dilute solution and a concentrated solution. The semipermeable membrane controls the flow of solute from the concentrated solution to the dilute solution thus bringing about conditions for equalizing the concentrations of solute on the two sides of the membrane to form an equilibrium. In reverse osmosis, pressure is deliberately applied to the more concentrated solution causing the flow of solvent in the opposite direction through the membrane, i.e., into the more dilute solution. In this way the liquid can be separated from solids and dissolved solids, decreasing the concentration of the solids and dissolved solids in the filtered fluid.
The wide spread use of reverse osmosis to produce potable water began in the early 1960's when Loeb and Sourirajan developed thin-skin cellulose acetate membranes for use in reverse osmosis systems. These cellulose acetate membranes provided much higher salt rejection (approaching 95%) and solvent flow than previously known reverse osmosis methods. Cellulose acetate membranes are also relatively inexpensive and are very tolerant of chlorine which is commonly used to eliminate bacteria in water. Since the 1960's the use of reverse osmosis has grown dramatically in waste water applications and industrial desalinization plants to produce drinking water from brackish and sea waters. More recently cellulose acetate membranes have been incorporated into consumer filtration systems to produce drinking water at the point of use. Matsuura, T., Synthetic Membranes and Membrane Separation Processes, CRC Press, (1994). Although cellulose acetate membranes greatly expanded the utilization of reverse osmosis treatment systems, such systems are still restricted by operational problems. For example, cellulose acetate membranes biodegrade readily.
Recently, thin film composite polyamide membranes have been developed that offer better performance than cellulose acetate membranes. These composite polyamide membranes exhibit salt rejection rates greater than 99.5% at pressures much lower than the pressures used for cellulose acetate membranes. Additionally, polyamide membranes reject silica, nitrates, and organic materials much better than cellulose acetate membranes. Because of the high performance of composite polyamide membranes, these membranes are used in high purity or ultrahigh purity water systems in pharmaceutical and electronics industries. However, just as cellulose acetate membranes exhibit a limiting characteristic (i.e., biodegradation) so do composite polyamide membranes. Composite polyamide membranes are susceptible to damage from chlorine.
As the technology for manufacturing composite polyamide and cellulose acetate membranes has progressed, new fields of filtration, called ultrafiltration (also called nanofiltration) and microfiltration have been created. Membranes based on polysulfone, polycarbonate, polypropylene, polyvinylidene difluoride and nylon have been developed for these applications.
For example, membranes used in hyperfiltration remove particles of 1-10 Angstrom units and include chemical compounds of about 180 to 15,000 molecular weights. Ultrafiltration filters particles of 30 to 1,100 Angstrom units that includes macromolecules of molecular weight of 10,000 to 250,000. Microfiltration which is mainly used to remove bacteria from solutions covers the range of 500 Angstrom to 20,000 Angstroms or 0.05 to 2 microns. (Lonsdale, H. K. "The Growth of Membrane Technology" Journal of Membrane Science, 10, p.80-81 (1982)). Unfortunately, these great strides in filtration have come at a cost, primarily in the form of bacteria contamination of filters and increased back pressures.
Bacteria contained in influent water may be arrested by reverse osmosis filters. In such a filter bacteria accumulate on the surface of the semipermeable membranes. Bacteria multiply every 30-60 minutes. Their growth is logarithmic and a single bacterial cell will result in 16 million bacteria in 24 hours. The explosive growth of bacteria results in fouling of the membrane which reduces the flow of water through the membrane and can adversely affect the filtering properties of the membrane. For example, bacteria build-up typically has an adverse affect on salt rejection in a reverse osmosis membrane. (Wes Byrne, Reverse Osmosis, Chapter 9- Biological Fouling). Fouled membranes require higher operating pressures which in turn increases operating costs.
In addition to reducing water quality and pressure, bacteria fouled membranes are difficult to clean. As a result of bacterial growth on the membrane, a gelatinous biofilm is formed on the upstream surface of the membrane which is very difficult to remove, except through use of strong chemical oxidants that damage the membrane. The biofilm protects the bacteria from the normal cleaning and sanitizing procedures and leads to a break through of bacteria across the membrane. This phenomena is not completely understood, since the pores of most reverse osmosis and ultrafiltration membranes are at least 2 to 4 orders of magnitude smaller than the bacterial cells. One possible explanation is that the bacterial cells exist in a dynamic state with continuous morphological changes occurring throughout the population. These bacteria then get more opportunities and time to find their way to an accommodating pathway through the membrane. Typically, bacteria are detected on the downstream side of the membrane in 48 to 72 hours. The downstream side of the membrane becomes discolored or black over time as the bacteria colonize on the downstream side of the membrane and form a biofilm that is difficult to remove. Such biological fouling can also lead to formation of localized extremes in pH that can damage the membrane.
The filter cartridges described in U.S. Pat. No. 5,762,797; application Ser. No. 08/877,080 and application Ser. No. 60/090,966 provide solutions to the problems created by bacteria buildup in reverse osmosis filters. By incorporating antimicrobial agents within various structures within the filter, water filters may be produced that are capable of removing and eliminating practically all microorganisms that may be present in the influent.
However, these filters, especially those with smaller pore sizes, create substantial back pressures in water delivery systems. In many countries the water pressure in municipal water lines is less than 60 psi. In such countries 0.1 to 0.45 micron rated filters, such as those described in U.S. Pat. No. 5,762,797, result in flow rates too low for practical operation. To address this problem the continuation application, Ser. No. 08/877,080, taught among other things, the use of a filter cartridge with semipermeable membranes having a nominal pore size of 0.75 microns. Increasing the nominal pore size increases the flow of the water through the filter cartridge without increasing back-pressures.
Unfortunately, increasing the nominal pore size of a filter also compromises the filter's ability to retain and deactivate bacteria. For example, some bacteria may slip through pores of 0.75 microns. In theory, it is preferable to approach a nominal pore size of 0.1 micron, because as the nominal pore size decreases, the higher the log reduction of bacteria and the better the performance of the filter cartridge as a bactericidal device.
Perhaps the primary factor limiting flow of water through the above described filters is the total surface area of the membrane through which water is able to pass or more specifically, the lack of surface area. When a semipermeable membrane is in the form of a flat sheet, as is typically utilized in a microfiltration filter cartridge, the maximum surface area is limited to the circumference of the plastic or activated carbon core over which it is wrapped. One method to increase surface area is to pleat the filter medium as is done in purely mechanical membrane filters, such as automobile oil filters. In the microfiltration context this solution is difficult to implement.
In short, a need exists for a reverse osmosis water filter that is capable of retaining and eliminating bacteria and allowing sufficient fluid flow and water pressure to be of practical use in water systems around the world.